The removal of particulates from a gas stream has long been a practice in a variety of industrial fields. Conventional means for filtering particulates and the like from gas streams include, but are not limited to, filter bags, filter tubes and filter cartridges. For convenience herein, the term "filter element" will be used to refer collectively to these types of filtration means.
Conventional filtration techniques utilize the filter element to stop particles through the depth of the element, and as the particles build up in and/or on the element, the filtration efficiency of the element is increased. After an amount of dust has caked on the surface of the filter element, the flow rate of gas through the element is reduced to a level where the bulk dust cake must be removed from the element, typically by some form of agitation, such as vibration or the like.
Filter elements are typically constructed from felts and/or fabrics made from a variety of materials, including polyesters, polypropylenes, aramids, glasses and fluoropolymers. Selection of the type of material used is typically based on the gas stream with which the filter element comes in contact, the operating conditions of the system and the type of particulate being filtered.
It is common in the industrial filtration field to characterize the type of filter element, such as the filter bag, by the method of cleaning. The most common types of cleaning techniques are reverse air, shaker and pulse jet. Reverse air and shaker techniques are considered low energy techniques.
The reverse air technique is a gentle backwash of air on a filter bag which collects dust on the interior. The back wash collapses the bag and fractures the dust cake which exits the bottom of the bag to a hopper.
Shaker mechanisms clean the filter cake that collects on the inside of a bag as well. The top of the bag is attached to an oscillating arm which creates a sinusoidal wave in the bag to dislodge the dust cake.
Pulse jet cleaning techniques employ a short pulse of compressed air that enters the interior top portion of the filter tube. As the pulse cleaning air passes through the tube venturi, it aspirates secondary air and the resulting air mass violently expands the bag and casts off the collected dust cake. The bag will typically snap right back to the support cage and go right back into service collecting particles.
Of the three cleaning techniques, the pulse jet is the most stressful on the filter media. However, in recent years industrial process engineers have increasingly selected pulse jet baghouses for dust collection applications because of: (1) smaller unit size (sometimes as much as 1/2 to 1/4 of the size of shakers and reverse air due to (a) higher volumetric airflow/cloth area ratios, and (b) on-line cleaning allows the unit to be designed at the design flow rate without the need for additional filter media area for off-line cleaning; (2) minimal number of moving parts; and (3) lower number of bags to replace when failed.
Referring to FIG. 8, a typical pulse jet cleaning sequence is shown. Inside hopper 120, the particulate laden gas stream (not shown) enters the hopper at inlet 122 and passes through filter bag 123. Tube sheet 125 inside hopper 120 prevents the gas stream from bypassing the filter bag. The filter bag 123 is kept open by support cage 126. The gas stream, after passing through the bag and out bag exit 129, exits the clean air compartment at outlet 127. In operation, particulate forms a dust cake 128 on the outside of the filter bag, as shown in the bag on the left of the figure. On cleaning to remove the filter cake, air from pulse pipe 130 enters the bag. This pulse of air 132 expands the bag, loosening the dust cake and thus causing particulate 131 to collect at the bottom of the hopper 120. As seen in the bag on the right of the figure, the pulse jet causes the filter bag to expand.
Polytetrafluoroethylene (PTFE) has demonstrated utility in many areas. As an industrial material, such as a filtration material, for example, PTFE has exhibited excellent utility in harsh chemical environments, which normally degrade many conventional metals and polymeric materials. PTFE is also usable over a broad temperature range, from as high as 260.degree. C. to as low as near -273.degree. C.
However, conventional non-porous PTFE materials possess insufficient porosity to be effective as filtration means, particularly in the case of unexpanded PTFE in sheet form. Alternative means have been developed, such as the formation of woven felts or mats of unexpanded PTFE fibers, whereby particles are trapped between the fibers in the weave. Limitations still exist in these materials, however, due at least in part to the non-porous nature of the PTFE.
A significant development in the area of particle filtration was achieved when expanded PTFE membrane was incorporated as a surface laminate on conventional filter elements. One example is taught in U.S. Pat. No. 4,878,930, directed to a filter cartridge for removing particles of dust from a stream of moving gas or air. Preferred filter media for the cartridge are felt or fabric composites containing a layer of porous expanded polytetrafluoroethylene membrane.
Use of expanded PTFE membrane greatly enhanced the performance of filter elements because the particles collected on the surface of the expanded PTFE, rather than in the depth of the elements as was occurring in the absence of the expanded PTFE layer. Several significant advantages were observed with these filter elements. First, the filtration efficiency of the elements was high immediately from the outset of the filtration process, and it was not necessary to "build up" a cake of particles to achieve high efficiency. Second, the elements lasted longer because particles were not getting into the backing fabric of the element and rubbing on the fibers to wear them out. Third, the cleaning energy needed to clean the particle cakes off of the elements was lower because the surface of the membrane was smooth and had a lower surface energy.
A filter bag made completely of expanded PTFE is described in U.S. Pat. No. 4,983,434, which shows an expanded PTFE membrane laminated to a felt of carded expanded PTFE staple fiber. This filter bag provides good pulse jet cleaning capabilities due to the strength and flexibility of the expanded PTFE, while also providing good heat resistance, chemical inertness and high air permeability.
In each of the cases described above incorporating expanded PTFE, the filter element comprises a membrane laminated to a backing material which purportedly provides support to the membrane to permit it to withstand the rigors of the filtration and cleaning processing. Conventional teachings in the field of filtration focused on the need for heavier support, or backing, materials to provide more durability to the filter element; however, the use of heavier support materials for higher strength and durability led to a trade-off with blocking more airflow through the filter and requiring more energy to clean the filter element.
For example, laminates weighing up to 22 ounces/square yard (745 g/square meter) were developed which provided longer life, but were heavy, bulky and required more energy to flex or clean the elements. Further drawbacks to such materials included, but were not limited to, high manufacturing costs due to the complex nature of the laminated elements, wear of the bags due to internal stresses between the laminated layers, the need for precision fitting of the elements in the filter assemblies in order to prevent movement of the filter element against the support, resulting in wear, and eventually failure, of the bag, difficulty in achieving effective cleaning, contamination due to particulation of the laminated media, larger quantities of material to dispose of after the filter bags wore out, and the need to accommodate the excess bulk of the laminated filter elements within the design of the filtration assemblies.
U.S. Pat. No. 5,074,896, issued Dec. 24, 1991, in the names of Baert et al., teaches a gas filtration device in a frame of a multiplicity of elongated porous PTFE membrane filter pouches supported within by individual frames and without by a cradle. Preferred filtration membranes are described as including porous PTFE, known under the designation of Gore-Tex.RTM., and expanded PTFE membranes. However, in the filtration systems taught by Baert et al., the filtration media made only from expanded PTFE membrane without a backing layer would not have sufficient durability to withstand the installation and operating conditions. Particularly, the expanded PTFE would be highly susceptible to wear or tear on the interior frame or cage made from e.g., metal, during filtering. Thus, the system described by Baert et al. suffers from significant limitations in performance.
U.S. Pat. No. 4,861,353, to Wyss, issued Aug. 29, 1989, is directed to a filter element and assembly including a tubular textile of filamentary PTFE useful to prolong the mechanical life of a filter material. The tubular element has a gas permeability of at least 1000 ft.sup.3 per ft.sup.2 per minute measured at a pressure of 1/2 inch of water. The tubular textile is placed over a filter cage and a filter bag is placed over the tubular textile to (1) prevent direct contact between the filter bag and the metal, and (2) lower the extent to which the filter is pressed into the interstices of the cage. However, the tubular textile taught by Wyss is limited to assemblies incorporating filter bags which are relatively inflexible. Particularly, conventional filter bags comprising a microporous membrane laminated to a backing material, are particularly suited for use with the tubular textile, as the flexing of the filter bag during the filtering operation results in no, or only minimal, contact with the exposed cage; however, the open weave of the tubular textile of filamentary PTFE taught by Wyss leaves exposed surfaces of the filter cage which could contact and damage filter bags incorporating flexible media. Thus, significant restrictions exist with respect to the filter bags which may be incorporated with the filtration assemblies taught by Wyss.
The novel filter elements of the present invention are designed to solve these problems and provide significant advantages over the filter elements of the prior art, as described in more detail herein.